September on the North Rim of the Grand Canyon. A female, well-liked, highly motivated, good to excellent physical condition, 21 years old, reports a nagging headache, unusual weakness and fatigue, lightheadedness, and nausea without vomiting. Midafternoon now, and she doesn't think she can continue backpacking today, and she has begun to question whether or not she can finish the expedition.

You notice anxiety--not pronounced, but present. Heart rate is 96. Respiratory rate is 20, unlabored. She is very sweaty. She reports unusual cramping in her lower extremities. She denies allergies, and reports no intake of medications. "Nothing like this has ever happened before," she states. She ate a light breakfast, and no lunch or snacks. She denies thirst, and claims to have drunk "at least three liters of water today."

What are you going to do? The leaders move her to shade and encourage hydration. Yep, it sounds like heat exhaustion. But if you treat it like heat exhaustion--just add water--you may harm this patient. She is not heat exhausted. She is hyponatremic.

At the Grand Canyon Clinic where Tom Myers, MD, practices his medical skills, hyponatremia as a diagnosis was virtually unheard of 10 years ago. Today, it accounts for more than 30 percent of the heat related complications he sees. Why? Because people are drinking lots of water--they got that message--but they aren't consuming adequate electrolytes. The formula for disaster is relatively simple: Salt loss in sweat exceeding salt intake plus water intake exceeding water loss equals lowered sodium level in the blood. When blood sodium gets too low, you have a case of hyponatremia.

"Of critical importance to the diagnosis," writes Dr. Myers, "is an accurate history." Relatively little salty food intake combined with relatively high fluid intake--say several liters in the last few hours--should make you highly suspicious. If the patient's urine output is clear and copious (urination occurring every few hours to several times per hour) combined with a lack of thirst, you'll draw closer to diagnosing hyponatremia. Heat exhausted patients typically have a low output of yellowish urine (urinating every 6-8 hours) combined with thirst.

Patients with mild to moderate symptoms and a normal mental status may be treated in the field: Rest in shade with no fluid intake and a gradual intake of salty foods while the kidneys reestablish a sodium balance. Once a patient develops hunger and thirst combined with normal urine output, the problem is solved. Restriction of fluids for someone who is well hydrated, fortunately, is harmless. Dr. Myers warns, however, that giving oral electrolyte replacement drinks, such as Gatorade, alone might damage the patient. These drinks are so low in sodium and so high in water the dilutional imbalance may be increased. Only a blood test confirms hyponatremia beyond doubt. If you just can't make up your mind--is it heat exhaustion or hyponatremia?--give the patient electrolyte replacement drinks and salty food, and monitor closely for improvement. Concerning patients with an altered mental status there is no question: They demand rapid evacuation to a medical facility. More severe symptoms of hyponatremia include a patient who is disoriented, irritable, and combative, which gives the problem the more common name of water intoxication. Untreated, the ultimate result will be seizures, coma, and death.

Prevention is a matter of being sensible, which is so often the case. Drink lots, yes, but eat salty foods regularly while exercising in heat. "Relying on electrolyte replacement drinks alone," writes Dr. Myers, "is absolutely ill advised."

Nothing's wrong with the electrolyte drinks, but don't forget to eat as well.

Buck Tilton, M.S., WEMT, is the author of numerous books and articles on wilderness medicine. He is the co-founder of the Wilderness Medicine Institute of NOLS and currently oversees curriculum development and international programming for WMI.